Generally, metallized film capacitors comprise two tightly wound sheets, wrapped around a core, with each sheet including a metallized layer on one side, with the metallization absent from two oppositely disposed margin layers. The opposite ends of the rolled capacitor are sprayed with a conductive metal which bonds with the sheet having a metallized end with leads attached to form the electrodes. The wound roll is usually surrounded by a dielectric fluid, with some impregnation occurring through the wound ends. Metallized film capacitors differ in the degree of impregnation from paper capacitors, because of the tightness of the winding and the lack of porosity of the film. While metallized film capacitors are preferred due to reduced labor requirements as opposed to paper or soggy foil capacitors, they have limited usefulness at higher voltages due to metal degradation.
Aliphatic or aromatic esters have been disclosed for use as dielectric capacitor impregnants, with the aromatic esters preferred in many applications. However, the aromatic esters have the disadvantage of exhibiting a high power factor at elevated temperatures, and suffer from degradation through hydrolysis. Various attempts have been made to overcome these disadvantages. In U.S. Pat. Ser. No. 3,754,173 to Eustance, an epoxide stabilized liquid aromatic ester is disclosed for use as an electrical capacitor impregnant in a paper or soggy foil capacitor, with the preferred ester being di(2-ethylhexyl) phthalate or DOP. The epoxide stabilizer increases the high temperature life of the capacitor by interacting with those chemical compounds found in or generated during operation, preventing those compounds from degrading the metallization. However, even stabilized by epoxides, metallized AC capacitors encased with aliphatic or aromatic esters continue to show a capacitance loss on life testing, which may be caused by corona, water, residual catalysts in the film, or incomplete impregnation of the wound section.
In U.S. Pat. Ser. No. 4,317,159 to Dequasie, a dielectric fluid is disclosed including 1-20% by weight of an aliphatic or aromatic isocyanate. While capacitance retention and life is increased, the presence and use of isocyanate compounds in the dielectric fluid involves environmental and safety risks due to the hazardous nature of these materials.
In U.S. Pat. Ser. No. 4,422,962 to Ciohanowsky, propylene glycol dielectric fluid is disclosed for impregnating metallized capacitors. In column 5, there is disclosed a comparison of epoxidized soybean oil with an aluminum metallized capacitor, showing capacitance changes of 5.2 and 4.1%. Such a comparison would lead one skilled in the art away from epoxidized soybean oil as a sole impregnant, as further illustrated in U.S. Pat. Ser. No. 4,642,731 to Shedigian, where glyceryl triacetate is mixed with epoxidized soybean oil. Shedigian states "that, the capacitance loss factor of epoxidized soybean oil is known to be considerably higher than that of glyceryl triacetate, for example 4.2% vs. 2.1% . . . " However, none of the references discuss capacitance loss in zinc or zinc alloy metallized capacitors, or note any differentiation between metallizations.
Consequently, the search continues for a high performance dielectric fluids for use in zinc or zinc alloy metallized capacitors which allows their utilization at higher voltages than previously achieved, providing enhanced performance, stability and life, while reducing environmental risks.